Conventionally, of water-soluble polymers including polyacrylic acid, polymaleic acid, etc., those having a low molecular weight have been suitably used as a dispersant for an inorganic pigment or metal ions, a scaling inhibitor, a detergent builder, etc. Known methods of obtaining such a water-soluble polymer having a low molecular weight are disclosed in, for example, 1 Japanese Laid-open Patent Application No. 38403/1989 (Japanese Official Gazette, Tokukaisho No. 64-38403, published on Feb. 8, 1989), and (Japanese Laid-open Patent Application No. 86125/1993 (Japanese Official Gazette, Tokukaihei No. 5-86125, published on Apr. 6, 1993).
According to the method disclosed in Gazette 1, when one or more than one kind of a water-soluble vinyl monomer (ethylenic monomer) is radical polymerized using a water-soluble azo radical polymerization initiator, sulfurous ions are provided in a range between 0.1 and 30 mol % based on a total weight of the monomer. The monomer used in this method is not especially limited as long as it is a water-soluble vinyl monomer, and by adopting this method, a water-soluble polymer having a molecular weight as low as ten to a hundred thousand molecular weight can be obtained at satisfactory reproductivity.
According to the method disclosed in Gazette 2, by subjecting acrylic acid or acrylate to aqueous solution polymerization while maintaining a pH of the reaction series in a range between 6 and 9 (under neutralized condition), a water-soluble polymer having 95 mol % or more of acrylic acid or acrylate can be obtained. An average molecular weight of the resulting polymer is in a range between 300 and 10,000, and a degree of dispersion is in a range between 1.3 and 2.3. Since the water-soluble polymer obtained by this method has a low molecular weight and a small variance in degree of dispersion (narrow molecular weight distribution), it exhibits a good dispersing ability, and therefore, can be suitably used as dispersants of various kinds, a detergent builder, etc.
Further, 3 Japanese Laid-open Patent Application No. 55407/1981 (Japanese Official Gazette, Tokukaisho No. 56-55407, published on May 16, 1981, or Tokukousho No. 60-24806) discloses a manufacturing method of a low-molecular-weight acrylate polymer. According to the method disclosed in Gazette 3, (A) alkali metal salt acrylate, (B) acrylamide or 2-hydroxyethyl (meth)acrylate, and (C) a hydrophilic monomer copolymerizible with the components (A) and (B) are subjected to aqueous solution polymerization at a specific ratio under the conditions: 10 l or more of air for 1 mole of the component (A), and a polymerization temperature at or below 80.degree. C.
The water-soluble acrylate polymer obtained by this method has a molecular weight ranging from 500 to 100,000 and a narrow molecular weight distribution. Also, this polymer is free of impurities and hardly colored. Moreover, 0.5-1.5 sulfonic acid group per molecule is introduced to an end terminal of a straight chain or a side chain of the water-soluble polymer. Hence, the water-soluble polymer has excellent dispersing ability and chelating ability, and therefore, can be suitably used as an inorganic pigment dispersant, a detergent builder, a detergent agent, or a scaling inhibitor.
Although it is not directed to the above water-soluble polymer, 4 U.S. Pat. No. 3,646,099 (US Official Gazette, published on Feb. 29, 1972) discloses a technique of manufacturing a low-molecular-weight polymer. A low-molecular-weight polymer disclosed in Gazette 4 is used for a conductive paint, which is obtained by polymerizing (meth)acrylonitrile with a hydrophobic monomer. Note that, however, 40 mol % or more of the hydrophobic monomer based on a total weight of the resulting polymer has to be included in raw materials. By adopting this method, a polymer having a molecular weight of 25,000 or smaller can be manufactured.
However, there remains a problem that none of the aforementioned methods can manufacture, in a sufficiently efficient manner, a low-molecular-weight water-soluble polymer having not only excellent dispersing ability and chelating ability, but also high resistance to gelling.
More specifically, with the technique disclosed in Gazette 1 or 2, it is very difficult to introduce a sulfonic acid group quantitatively to an end terminal or a side chain of the polymer. Thus, the resulting water-soluble polymer can not attain excellent dispersing ability or chelating ability, and therefore, can not be used optimally as a dispersant, a detergent builder, a detergent agent, or a scaling inhibitor.
On the other hand, with the technique disclosed in Gazette 3, sulfonic acid groups can be introduced to the resulting water-soluble polymer to some extent. However, since acrylate (alkali metal salt) is used as the monomer in this technique, a polymerization reaction series is almost completely neutralized.
If polymerization takes place in such a neutralized state using acrylate, when a solid concentration of the reaction series is increased, a viscosity of an aqueous solution of the reaction series increases sharply as the polymerization proceeds, and accordingly, a molecular weight of the resulting polymer tends to increase markedly. For this reason, with the technique disclosed in Gazette 3, a low-molecular-weight polymer can not be manufactured by polymerizing a monomer under a high-concentration condition, thereby posing a problem that a manufacturing efficiency is reduced.
Further, with the technique disclosed in Gazette 4, a low-molecular-weight polymer can be manufactured in a satisfactory manner, but since 40 mol % or more of a hydrophobic monomer has to be used, the resulting low-molecular-weight polymer is insoluble to water. In other words, the technique disclosed in Gazette 4 is directed to a technique of manufacturing a low-molecular-weight polymer for a conductive paint, and merely using a water-soluble monomer can not yield a water-soluble polymer in a satisfactory manner.
Also, in order to be used suitably as a dispersant, a scaling inhibitor, a detergent builder, etc., it is preferable that the low-molecular-weight water-soluble polymer has not only excellent dispersing ability and chelating ability, but also high resistance to gelling, namely properties not turning into gel. However, a water-soluble polymer having high resistance to gelling can not be obtained with any of the techniques disclosed in aforementioned Gazettes, and as a consequence, a water-soluble polymer having sufficiently high abilities for use as the dispersant, scaling inhibitor, detergent builder, etc. can not be obtained.
On the other hand, it has been known that an (acrylic acid/acrylate-maleic acid/maleate) copolymer having a number of carboxyl groups exhibits excellent chelating function and dispersing function, and therefore, has been used in diversified fields as a detergent builder, a scaling inhibitor, an inorganic pigment dispersant, a fiber treatment agent, a chelating agent, etc.
When used as the detergent builder, desired basic abilities of the copolymer are a calcium ion trapping ability and a clay dispersing ability. The calcium ion trapping ability is used to determine an amount of calcium ions the copolymer can trap in a series where calcium ions exist exceedingly. On the other hand, the clay dispersing ability is indicated as turbidity of a supernatant obtained by mixing clay and the copolymer with an aqueous solution of a specific chemical composition and dispersing the clay in the aqueous solution, and the higher the turbidity, the better the clay dispersing ability of the copolymer. Also, in general, the greater a molecular weight, the better the calcium ion trapping ability and the lower the clay dispersing ability. On the other hand, the smaller a molecular weight, the better the clay dispersing ability and the lower the calcium ion trapping ability. Thus, it has been very difficult to improve both the abilities in good balance, and many solution methods have been proposed to date.
For example, PCT Unexamined Patent Publication No. WO95/03342 published on Feb. 2, 1995 (Japanese Patent No. 2574144) discloses a maleic acid copolymer having a calcium ion trapping ability of 300 mgCaCO.sub.3 /g or greater and a clay dispersing ability of 1.2 or greater, and a manufacturing method thereof. According to the disclosure of this Patent, the clay dispersing ability was measured by using clean water of Himeji City, Japan. Most of water supplied in Japan, including the above clean water of Himeji City, has a relatively low hardness with 20-50 ppm in CaCO.sub.3 conversion, and the conditions for the dispersing ability are not as strict as those for the calcium ion trapping ability. Hence, Japanese Patent No. 2574144 mainly discusses the calcium ion trapping ability.
However, water supplied in many regions outside Japan has a high hardness. For example, a hardness of water supplied in the Unites States and China is as high as 200 ppm (in CaCO.sub.3 conversion). The copolymer disclosed in Japanese Patent No. 2574144 can exhibit an excellent dispersing ability when used in low hard water, such as water supplied in Japan, but can not do so when used in high hard water. Compared with improvements of the dispersing ability in low hard water, it is far more difficult to improve the clay dispersing ability in high hard water, and the copolymer disclosed in Japanese Patent No. 2574144 focused on the improvement of the calcium ion trapping ability can not overcome the above problem.
Besides Japanese Patent No. 2574144, many manufacturing methods of an (acrylic acid/acrylate-maleic acid/maleate) copolymer have been proposed. For example, Japanese Laid-open Patent Application No. 235313/1988 (Japanese Official Gazette, Tokukaisho No. 63-235313, published on Sep. 30, 1988) discloses a manufacturing method of a maleic acid copolymer which contains acrylic acid/acrylate and maleic acid in a mole ratio ranging from 100/200 to 100/25 and has a sulfonic acid group at an end terminal. More specifically, disclosed in this Gazette is a method of copolymerizing acrylic acid/acrylate with maleic acid in an aqueous solution of a pH maintained in a range between 2.5 and 6.5 using a polymerization initiator produced by reacting sulfurous acid or bisulfurous acid, or a salt thereof with an oxidant. The resulting copolymer exhibits an excellent scaling inhibiting ability, but when used as a detergent, there arises a problem that it exhibits a poor clay dispersing ability due to its high ratio of maleic acid, especially with use in high hard water.
Also, Japanese Laid-open Patent Application No. 67706/1983 (Japanese Official Gazette, Tokukaisho No. 58-67706, published on Apr. 22, 1983) discloses a method of copolymerizing unsaturated monocarboxylic acid/monocarboxylate, such as acrylic acid, with unsaturated dicarboxylic acid/dicarboxylate, such as maleic acid, in a weight ratio of 90-40/10-60 in the presence of a water-soluble initiator. However, according to examples disclosed therein, maleic acid is used at a high ratio and a hydrogen peroxide alone is used as the initiator. Hence, when used as a detergent, there arises a problem that the resulting copolymer exhibits a poor clay dispersing ability, especially with use in high hard water.